Heat 20 g of butter and 2 table spoons of olive oil in a large pan and fry the onions on a medium heat until golden brown. Add the thyme leaves and cook for a couple of minutes.
Remove the onions and set aside in a bowl to cool. Add the remaining butter, breadcrumbs, hazelnuts and parsley and mix well. Season with salt and pepper.
Season the lamb filets with salt and pepper. Heat the remaining olive oil in a pan and fry the meat on a high heat from all sides to seal it. Place the meat in a flat oven proof dish. Spread the hazelnut mixture on top.
Place the lamb in a preheated oven for about 210 degrees Celsius. Bake for about 20 minutes until golden brown.